Block Syntax (1)

1. Why block is used.

We know that there are three ways to communicate between objects and objects: 1, Agent-agreement, 2, notice, 3, block. There are three ways to achieve the decoupling between objects. The difference is: the communication mode of notification is 1 to many, the agent, Block communication mode is 1 to 1.

Introduction to 2.Block.

• Block is a new syntactic structure after iOS4.0, and also becomes a "closure".
• SDK4.0 New API heavily uses block
• Block is an anonymous function code block that can be passed as a parameter to other objects.

3. Use of block

// int is the return value; Sumblock is the block variable type; (int x,int y) The parentheses are the parameters.  int(^sumblock) (int x,int y);

　

1 //Create Myblock2- (void) Createmyblock {3     //defining a block variable4     int(^sumblock) (intXinty);5     //implement block, and assign values6Sumblock = ^ (intXinty) {7         returnX +y;8     };9     //declares the Myblock variable, global {Int (^myblock) (int x,int y);}

Ten     Myblock = Sumblock;  One     NSLog (@ "%d", Myblock ());  --}

4. Create block

Block has the following 4 ways:

1 //block is an anonymous function and cannot be called by function name, so it doesn't make sense to use it alone2 //1. No parameter no return value--the same effect as the direct printing data, meaningless3^(){4NSLog (@"No parameter no return value");5     }();6     7 //2. No return value with parameters8^(inta,nsstring*text) {9NSLog (@"%d \%@", a,text);Ten}(Ten,@"There are no return values for parameters"); One      A //3. There is no parameter for return value -nsstring* A = ^(){ -         return @"no parameter has return value"; the     }(); -NSLog (@"%@", a); -      - //4. Parameters have a return value +     //defining a block variable -     int(^sumblock) (intXinty); +     //implement block, and assign values ASumblock = ^ (intXinty) { at         returnX +y; -     }; -NSLog (@"%d", Sumblock (Ten,Ten)); -}

1 //The parameter has a return value (The return result is 6)2     int(^powefblock) (intXinty);3Powefblock = ^ (intXinty) {        4         returnX *y;5     };6NSLog (@"powef =%d", Powefblock (2,3));

5. Bloc and variables

• Variables can be divided into global variables and local variables by function.
• These two variables can be used in a block, but there are different management mechanisms.

Referencing local variables

• When a block refers to a local variable, the change amount is encoded as a constant in block blocks.
• Local variables need to be modified in block by using the __block modifier.

1 //If an external variable is used inside the block code, number becomes constant2     intNumber =Ten;3^(){4  　　　　 //Number = 20; Direct Error---Can not be modified, has become constant5NSLog (@"%d", number);6     }();7 8 //-- changes in the interior-9 //add a __block declaration before the variableTen__blockintnumber_1 =Ten; One^(){ Anumber_1 = -; -NSLog (@"%d", number_1); -}();

• referencing external variables

#import <Foundation/Foundation.h>@interface  myobject:nsobject{        // block references external member variables    int _number;} -(void) log; @end

1- (void) Log {2    3 //block references external member variables4_number = About;5NSLog (@"---%ld--", Self.retaincount); 6 //use block to modify the Obj object so that obj is used in block and is not retain. 7 //obj cannot have ownership of self block does not hold self object8__block myobject* obj =Self ; 9^(){Ten         //determine if obj exists One         if(!obj) { A             return ; -         } -         //gets the member variable through obj; the print result is 1. the         intA = obj_number; -NSLog (@"---%d--", a); -NSLog (@"*****%ld****", self.retaincount); -     }(); +      -}

6. Block's memory management

• When a local Objective-c object is referenced within a block, the object is retain.
• If the local variable uses the __block adornment, it will not be retain.  

1 //--------Reference to an external obj object----------2 //will not be retain, i.e. Obj1.retaincount is always 13 //__block nsobject* obj1 = [[NSObject alloc] init];4 //Obj1.retaincount will change to 2 after the delay call5nsobject* obj1 =[[NSObject alloc] init];6NSLog (@"%ld", obj1.retaincount);7     8     //Delay Call9Dispatch_after (Dispatch_time (Dispatch_time_now, (int64_t) (2* nsec_per_sec)), Dispatch_get_main_queue (), ^{        TenNSLog (@"%ld", obj1.retaincount); One});

• Block itself can copy and release as objects
• After the block is created, the memory is allocated on the stack, and after the copy method is called, the block is moved to the heap.
• When the block is declared as a global variable, the block's copy method should be called.

7. Circular References

• When referencing an instance variable in block (dot syntax), the instance variable is retain.
• such as the rules can easily lead to circular references, resulting in

Readers are welcome to check, if reproduced, please indicate the source.

Block Syntax (1)